mtd: spi-nor: Fix SPI NOR acronym

The correct terminology is serial NOR flash or SPI NOR.
s/SPI-NOR/SPI NOR and s/spi-nor/SPI NOR across the subsystem.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>